vkd3d-shader: Use iterators everywhere, part 4/M (alt. dimension).
Since I am working on replacing the instruction array with the rbseq.h structure, I inevitably need to replace direct accesses to the vkd3d_shader_instruction_array with iterators too, so I am also sending iterator patches. I hope I am not overlapping too much with work that @giomasce's might have on his backlog though.